Chemical-mechanical polishing ("CMP") processes remove material from the surface of the wafer in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a polishing pad in the presence of a slurry under controlled chemical, pressure, velocity, and temperature conditions. The slurry solution generally contains small, abrasive particles that abrade the surface of the wafer, and chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a continuous phase matrix material such as polyurethane. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and by the chemicals (chemical removal) in the slurry.
FIG. 1 schematically illustrates the conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. An under-pad 25 is typically attached to the upper surface 22 of the platen 20, and the polishing pad 40 is positioned on the under-pad 25. In conventional CMP machines, a drive assembly 26 rotates the platen 20 as indicated by arrow A. In other existing CMP machines, the drive assembly 26 reciprocates the platen 20 back and forth as indicated by arrow B. The motion of the platen 20 is imparted to the pad 40 through the under-pad 25 because the polishing pad 40 frictionally engages the under-pad 25. The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free floating wafer carrier, but an actuator assembly 36 is preferably attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows C and D, respectively.
In the operation of the conventional CMP machine 10, the wafer 12 faces downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 12 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 12. CMP processes typically remove either conductive materials or insulative materials from the surface of the wafer to produce a flat, uniform surface upon which additional layers of devices may be fabricated.
When a conductive layer is polished from a wafer, the CMP processes must accurately stop polishing the wafer at a desired endpoint. Conductive layers are typically deposited over insulative layers to fill vias or trenches in the insulative layer and form electrical interconnects between device features on the wafer. To electrically isolate the interconnects from one another, it desirable to stop the CMP process below the top of the insulative layer and above the bottom of the conductive material in the vias and trenches. If the CMP process is stopped before the desired endpoint ("under-polishing"), then the interconnects will not be electrically isolated from one another and shorting will occur in the circuit. Conversely, if the CMP process is stopped after the desired endpoint ("over-polishing"), then interconnects may be completely removed from the wafer. Therefore, to avoid serious defects in a wafer, it is highly desirable to stop the CMP process at the desired endpoint.
U.S. Pat. No. 5,433,651 to Lustig et al. discloses an apparatus and a method for determining the endpoint of a CMP process in which a laser beam passes through a window in the polishing pad and impinges upon the polished surface of the wafer. The laser beam scans across the surface of the wafer, and a photosensor senses the intensity of the beam that reflects from the wafer. Conductive materials, such as aluminum, have a reflectivity index of approximately 90%, while insulative materials, such as boro-phosphate silicon glass ("BPSG"), have a reflectivity index of approximately 35%. At the endpoint of the CMP process, therefore, the intensity of the reflected beam alternates between that of the conductive material and the insulative material as the laser beam scans across the wafer. The Lustig et al. patent discloses that the endpoint of the CMP process is detected when the intensity of the reflected beam changes from that of the conductive material to the average intensity of the conductive and insulative materials.
One problem with the method of determining the endpoint of the CMP process disclosed in the Lustig et al. patent is that it may not accurately detect the endpoint on wafers that have small "critical areas." The critical areas are typically depressions on the surface of the wafer that are the last point on the wafer from which the conductive material is removed by CMP processing. The location and size of the critical areas is a function of the circuit design and the uniformity of the polishing rate across the surface of the wafer. As a result, the critical areas vary from one type of die to another, and they typically occupy a minuscule portion of the wafer surface. The method disclosed in the Lustig et al. patent may not detect the status of the CMP process at many critical areas on the wafer because the critical areas occupy such a small percentage of the wafer's surface that the few reflective signals generated by the critical areas do not statistically impact the overall average reflectivity of the substantially larger number of reflective signals from the interconnects. Thus, even if the Lustig et al. patent recognized the problem of critical areas, it may not accurately detect the endpoint of the CMP process at critical areas on the wafer.
In light of the problems with detecting the endpoint of the CMP process at critical areas on the wafer, it would be desirably to develop a method that quickly and accurately detects the endpoint of CMP processing at predetermined critical areas on a semiconductor wafer.